The invention relates to a method and apparatus for increasing a transmission performance of a hybrid wavelength division multiplexing system, and in particular to a method and apparatus for reducing cross-phase modulation in hybrid QPSK-OOK dense wavelength division multiplexing systems (DWDM).
In optical networks optical signals are transported via optical fibers from an optical transmitter to an optical receiver. In many optical networks it is necessary that optical signals of different types do co-propagate through the same optical fiber. In wavelength division multiplexing systems it is possible that optical amplitude modulated signals are travelling along the same fiber as phase modulated signals. For example, an On/off keying signal can co-propagate with a PDM (Polarization Division Multiplexing)-QPSK-(Quadrature Phase Shift keying) signal. Furthermore, optical signals propagating on the same fiber can have different modulation formats. This kind of a mixed rate, mixed format optical network can be referred to as a hybrid system.
FIG. 1 shows a spectrum of an optical signal travelling along a fiber of the optical network wherein the signal is composed of a phase modulated signal and an amplitude or intensity modulated signal. The spectrum of FIG. 1 depicts a 100 Gb/sec PDM-QPSK channel with a frequency of 193.4 Terahertz (THz) surrounded by 10 Gb/sec OOK channels in a standard ITU 50 GHz grid. The 100 Gb/sec PDM-QPSK signal is a phase modulated signal generated by a nested Mach-Zehnder modulator also known as quadrature modulator whereas the 10 Gb/sec OOK signal forms an intensity modulated signal generated by an optical amplitude modulator. For the phase modulated signal such as the 100 G PDM-QPSK signal the worst case cross-phase modulation XPM occurs when the neighbouring channels are intensity modulated such as the amplitude modulated 10 G OOK signals co-propagating through the same optical fiber. This type of legacy system employs an inline optical dispersion compensation using a dispersion compensation fiber DCF which is necessary for the 10 G OOK signals and usually cannot be removed from the optical network without interruption of traffic signals. The inline dispersion compensation achieved by means of the dispersion compensation fiber DCF further increases the cross-phase modulation XPM by removing or reducing a walk-off between channels due to chromatic dispersion. Cross-phase modulation XPM is a non-linear optical effect where one wavelength of light affects the phase of another wavelength of light through the so-called optical Kerr effect. This cross-phase modulation has the disadvantage that it can lead to interchannel crosstalk in wavelength division multiplexing systems. It further can produce amplitude and timing jitter.
To reduce cross-phase modulation XPM different reduction mechanisms have been proposed. A conventional way to reduce cross-phase modulation is to reduce the power of the 10 G channels in comparison to the power of the 100 G channels. This, however, requires a complex software algorithm to communicate with intermediate nodes of the network and a control power per channel at all amplifiers and ROADM (Reconfigurable Optical Add-drop Multiplexer) nodes within the optical network.
Further, it has been proposed to insert channel separation devices such as ROADM, interleavers or multiplexing/demultiplexing devices to demultiplex WDM channels and transmit them through different paths within the same node of the network with adequate path length differences to introduce a differential delay between the channels. Another conventional mechanism is the insertion of a fiber Bragg-grating DCM instead of the dispersion fiber based DCMs to increase the walk-off between the 10 Gbit/sec On/Off keying (OOK) amplitude modulated signal and the 100 Gbit/sec PDM-QPSK channels. Further, it was proposed to insert additional devices to create PMD. All these proposals have the disadvantage that they depend on specific devices being present in the network or being added into the system for the sole purpose of reducing cross-phase modulation XPM. Moreover, the insertion of additional devices to create PMD does reduce a transmission distance of the 10 G OOK channels. Another disadvantage of these conventional mechanisms is that they do not allow an in-service upgrade of 10 G to 100 G channels.
Accordingly, it is an object of the present invention to provide a method and apparatus for increasing a transmission performance of a hybrid wavelength division multiplexing system not having the above-mentioned disadvantages of the conventional mechanisms.